In comparison with conventional intensity modulation-direct detection or phase modulation-delayed interference detection technologies, coherent optical communication technology can tolerate lower optical signal-to-noise ratio,. compensate linear damage by using digital signal processing technology, and achieve higher spectral usability. These advantages of the coherent optical communication technology satisfy the demand on higher single wavelength rate and higher spectral efficiency of next-generation optical communication networks, and hence arouse great interests in people.
In a coherent optical receiver, signal light is mixed with local oscillation light generated by a local laser, and a baseband electric signal capable of reflecting electric field envelope of the signal light is then obtained after photoelectric transformation. The original transmission signal can be recovered by performing sampling quantization and digital signal processing on the baseband electric signal. In practical application, however, it is impossible to guarantee that the frequency of the laser in the transmitter and the frequency of the laser in the receiver are identical, so that there is nonzero intermediate frequency, namely frequency offset, in the baseband electric signal. At present, aberration in the nominal emission frequency of a commercially available laser is up to ±2.5 GHz, and in such a circumstance, frequency offset in an actual system can reach as high as ≅5 GHz. Consequently, frequency offset in the coherent optical receiver must be detected (with the detecting range reaching at least ±5 GHz), so as to control or compensate the frequency offset. On the other hand, the optical communication system continuously operates at a very high signal rate, there is hence a demand for low complexity and high precision on the frequency offset detecting method.
FIG. 1 shows the structure of a known coherent optical receiver. As shown in FIG. 1, a 90-degree optical mixer 103 mixes a received optical signal 101 with a continuous light 102 outputted from a local oscillation laser 104 and outputs the same to balanced photoelectric detectors 105 and 106. The balanced photoelectric detectors 105 and 106 convert the optical signal respectively into a cophase component and a quadrature component of baseband electric signals. Subsequently, a discrete digital baseband complex signal 109 (sometimes also referred to as complex signal discrete sequence, complex signal sequence or complex signal sampling sequence) is obtained from the two branches of baseband electric signals after sampling and quantizing by analog-to-digital converters 107 and 108. The local oscillation laser 104, the balanced photoelectric detectors 105 and 106, and the analog-to-digital converters 107 and 108 constitute a front-end processing section of the digital coherent optical receiver. An electric equalizer 110 performs equalized filtering on the complex signal sequence 109 to compensate linear transmission damage, and chromatic dispersion of a signal 111 outputted thereby is almost completely compensated. A follow-up digital signal processing module 116 performs carrier wave phase recover, differential decoding and the like on the signal 111 to thereby recover the transmitted data 117. Since there is frequency offset between the carrier wave and the local oscillation laser, one branch is divided from the complex signal 109 to the frequency offset detecting apparatus 112 for frequency offset detection there. If it is desirable to prevent such damage as the chromatic dispersion from affecting frequency offset detection, it is also possible to divide one branch from the signal 111 as outputted from the equalizer as input into the frequency offset detector. The frequency offset detecting apparatus 112 performs corresponding processing on the signal 109 (or the signal 111) to obtain a detection signal 113, and uses it for frequency offset control. There are two working modes for frequency offset control. The first mode is the feedback control mode, whereby the frequency offset detecting apparatus 112 outputs the detection signal 113 to a local oscillation controller 114, and the local oscillation controller 114 converts the frequency offset detection signal 113 into a local oscillation frequency control signal 115, so as to tune the output frequency of the local oscillation laser as consistent with the signal carrier frequency. Another mode is the feed-forward compensation mode, whereby the frequency offset detecting apparatus 112 outputs the detection signal 113 to the digital signal processing module 116, and the digital signal processing module 116 compensates frequency offset in the signal 111 in the digital domain in accordance with the frequency offset detection signal 113.
FIG. 2 illustrates a frequency offset detecting apparatus proposed by Andreas Leven et al. (“Frequency Estimation in Intradyne Reception”, IEEE Photonics Technology Letters, Volume: 19, No. 6, March 15, pages 366-368). The complex signal 109 (or 111) that enters the frequency offset detector 112 is divided into two branches, one of which is connected to a register 201, and another one is connected to a multiplier 204. The register 201 and a complex conjugate calculator 202 perform delaying and conjugating operations on the complex signal 109 (or 111) to obtain a signal 203, and output it to another input terminal of the multiplier 204. The multiplier 204 multiplies the complex signal 119 (or 111) with the signal 203, and outputs the result to a quartic calculator 205. This procedure removes influence of the phase noise of the signal on frequency offset detection. Possible values of a QPSK modulated signal might be ±π/4, ±3π/4, then the possible values of the modulation information of the signal outputted by multiplier 204 are 0, ±π/2, ±π, ±3π/4. The modulation information is removed after passing through the quartic calculator, but the frequency offset becomes four times as much at the same time. A summator 206 functions as an averager to reduce influence of additive noise on frequency offset detection. Finally, a ¼ argument calculator 207 performs ¼ argument operation on the output of the summator 206 and outputs the frequency offset detection signal 113. This detection signal is a signal phase gain introduced by frequency offset in one sampling period.
This method is problematic in two aspects. Firstly, since the output range of the ¼ argument calculator is [−π/4, +π/4], the frequency offset range capable of being detected by the method is [−Rs/8, +Rs/8], where Rs indicates symbol rate. The highest symbol rate achievable in current optical transmission is 20 G symbol/second. Taking example of such a system, the frequency offset range estimable by this method is still only [−2.5 GHz, +2.5 GHz], which is only half of the required detection range of [−5 GHz, 5 GHz]. Secondly, the method includes not only multiplication operation of complex numbers but also quartic operation of complex numbers, and such computational complexity is by far higher than that of addition, subtraction and logic operations of real numbers. Insofar as the current digital signal processing technology is concerned, it is almost impractical to process symbols having rates of up to 10 G symbol/second or 20 G symbol/second in the optical transmission system with such high computational complexity.